MAX3738ETG-T [MAXIM]
Interface Circuit, BIPolar, 4 X 4 MM, QFN-24;
| 型号： | MAX3738ETG-T |
| 厂家： | 
MAXIM INTEGRATED PRODUCTS |
| 描述： | Interface Circuit, BIPolar, 4 X 4 MM, QFN-24 接口集成电路 |
| 文件： | 总16页 (文件大小：359K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
19-3162; Rev 2; 12/08
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
MAX738
General Description
Features
The MAX3738 is a +3.3V laser driver designed for mul-
tirate transceiver modules with data rates from
155Mbps to 4.25Gbps. Lasers can be DC-coupled to
the MAX3738 for reduced component count and ease
of multirate operation.
♦ Single +3.3V Power Supply
♦ 47mA Power-Supply Current
♦ 85mA Modulation Current
♦ 100mA Bias Current
Laser extinction ratio control (ERC) combines the features
of automatic power control (APC), modulation compensa-
tion, and built-in thermal compensation. The APC loop
maintains constant average optical power. Modulation
compensation increases the modulation current in pro-
portion to the bias current. These control loops, com-
bined with thermal compensation, maintain a constant
optical extinction ratio over temperature and lifetime.
♦ Automatic Power Control (APC)
♦ Modulation Compensation
♦ On-Chip Temperature Compensation
♦ Self-Biased Inputs for AC-Coupling
♦ Ground-Referenced Current Monitors
♦ Laser Shutdown and Alarm Outputs
♦ Enable Control and Laser Safety Feature
The MAX3738 accepts differential data input signals.
The wide 5mA to 60mA (up to 85mA AC-coupled) mod-
ulation current range and up to 100mA bias current
range, make the MAX3738 ideal for driving FP/DFB
lasers in fiber optic modules. External resistors set the
required laser current levels. The MAX3738 provides
transmit disable control (TX_DISABLE), single-point
fault tolerance, bias-current monitoring, and photocur-
rent monitoring. The device also offers a latched failure
output (TX_FAULT) to indicate faults, such as when the
APC loop is no longer able to maintain the average
optical power at the required level. The MAX3738 is
compliant with the SFF-8472 transmitter diagnostic and
SFP MSA timing requirements.
Ordering Information
PART
TEMP RANGE
-40°C to 85°C
-40°C to 85°C
PIN-PACKAGE
24 Thin QFN-EP*
24 Thin QFN-EP*
MAX3738ETG
MAX3738ETG+
+Denotes a lead(Pb)-free/RoHS-compliant package.
*EP = Exposed pad.
Pin Configuration
The MAX3738 is offered in a 4mm x 4mm, 24-pin thin
QFN package and operates over the extended -40°C to
+85°C temperature range.
TOP VIEW
Applications
Multirate OC-3 to OC-48 FEC Transceivers
24 23 22 21 20 19
1
2
3
4
5
6
MODTCOMP
18
17
MD
V
CC
V
CC
Gigabit Ethernet SFF/SFP and GBIC
Transceivers
IN+
IN-
16 OUT+
MAX3738
OUT-
15
14
13
1Gbps/2Gbps/4Gbps Fibre Channel SFF/SFP
and GBIC Transceivers
V
CC
V
CC
*EP
BIAS
TX_DISABLE
7
8
9
10 11 12
Typical Application Circuit appears at end of data sheet.
*THE EXPOSED PADDLE MUST BE SOLDERED TO SUPPLY
GROUND ON THE CIRCUIT BOARD.
PIN1 INDICATED BY + ON LEAD-FREE PACKAGE.
________________________________________________________________ Maxim Integrated Products
1
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
ABSOLUTE MAXIMUM RATINGS
Supply Voltage V ...............................................-0.5V to +6.0V
CC
OUT+, OUT-, BIAS Current.............................-20mA to +150mA
IN+, IN-, TX_DISABLE, TX_FAULT, SHUTDOWN,
BC_MON, PC_MON, APCFILT1, APCFILT2,
MD, TH_TEMP, MODTCOMP, MODBCOMP,
Continuous Power Dissipation (T = +85°C)
A
24-Pin TQFN (derate 27.8mW/°C above +85°C) .......1805mW
Operating Junction Temperature Range...........-55°C to +150°C
24-Pin TQFN (derate 27.8mW/°C above +85°C) ........1805mW
Storage Temperature Range.............................-55°C to +150°C
MODSET, and APCSET Voltage.............-0.5V to (V
+ 0.5V)
CC
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
MAX738
ELECTRICAL CHARACTERISTICS
(V
CC
= +2.97V to +3.63V, T = -40°C to +85°C. Typical values are at V
= +3.3V, I
= 60mA, I = 60mA, T = +25°C, unless
MOD A
A
CC
BIAS
otherwise noted.) (Notes 1, 2)
PARAMETER
POWER SUPPLY
SYMBOL
CONDITIONS
MIN
TYP
MAX
UNITS
Supply Current
I
(Note 3)
f ꢀ 1MHz, 100mA
47
33
60
mA
dB
CC
Power-Supply Noise Rejection
I/O SPECIFICATIONS
Differential Input Swing
PSNR
(Note 4)
P-P
V
DC-coupled, Figure 1
0.2
1.7
2.4
V
ID
P-P
V
V
CC
-
Common-Mode Input
V
CM
V
/ 4
ID
LASER BIAS
Bias-Current-Setting Range
Bias Off Current
1
100
0.1
95
mA
mA
TX_DISABLE = high
Bias-Current Monitor Ratio
LASER MODULATION
I
/ I
68
79
65
mA/mA
BIAS BC_MON
Modulation Current-Setting
Range
I
(Note 5)
5
85
80
mA
MOD
20% to 80%
(Notes 6, 7)
Output Edge Speed
5mA ꢀ I
ꢀ 85mA
ps
%
MOD
Output Overshoot/Undershoot
Random Jitter
With 1pF between OUT+ and OUT-
(Notes 6, 7)
±6
0.62
18
1.3
40
ps
RMS
2.7Gbps, 5mA ꢀ I
ꢀ 85mA
MOD
1.25Gbps, 5mA ꢀ I
ꢀ 85mA
ꢀ 85mA
ꢀ 85mA
20
41
MOD
MOD
MOD
Deterministic Jitter (Notes 6, 8)
ps
P-P
622Mbps, 5mA ꢀ I
155Mbps, 5mA ꢀ I
24
46
45
100
±600
±480
±20
±15
0.1
5mA ꢀ I
ꢀ 10mA
±175
±125
Modulation-Current Temperature
Stability
MOD
(Note 6)
ppm/°C
10mA ꢀ I
ꢀ 85mA
MOD
5mA ꢀ I
ꢀ 10mA
15ꢀ load,
MOD
Modulation-Current-Setting Error
Modulation Off Current
%
T
A
= +25°C
10mA < I
ꢀ 85mA
MOD
TX_DISABLE = high
mA
AUTOMATIC POWER AND EXTINCTION RATIO CONTROLS
Monitor-Diode Input Current
Range
I
Average current into the MD pin
18
1500
μA
MD
MD Pin Voltage
MD Current Monitor Ratio
1.4
V
I
/ I
0.85
0.93
1.15
mA/mA
MD PC_MON
2
_______________________________________________________________________________________
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
MAX738
ELECTRICAL CHARACTERISTICS (continued)
(V
CC
= +2.97V to +3.63V, T = -40°C to +85°C. Typical values are at V
= +3.3V, I
= 60mA, I = 60mA, T = +25°C, unless
MOD A
A
CC
BIAS
otherwise noted.) (Notes 1, 2)
PARAMETER
APC Loop Time Constant
APC Setting Stability
SYMBOL
CONDITIONS
= 0.01μF, ꢁI / ꢁI
MIN
TYP
3.3
MAX
UNITS
μs
C
= 1/70
APC_FILT
MD
BIAS
(Note 6)
= +25°C
±100
±480
±15
ppm/°C
%
APC Setting Accuracy
T
A
I
Compensation-Setting
MOD
K
K = ꢁI
/ ꢁI
0
0
1.5
1.0
mA/mA
mA/°C
°C
MOD
BIAS
Range by Bias
I
Compensation-Setting
MOD
TC
TC = ꢁI
/ ꢁT (Note 6)
MOD
Range by Temperature
Threshold-Setting Range for
Temperature Compensation
T
TH
(Note 6)
+10
+60
LASER SAFETY AND CONTROL
Bias and Modulation Turn-Off
Delay
C
= 0.01μF, ꢁI
= 0.01μF, ꢁI
/ ꢁI
/ ꢁI
= 1/80
= 1/80
APC_FILT
MD
BIAS
5
μs
(Note 6)
Bias and Modulation Turn-On
Delay
C
APC_FILT
(Note 6)
MD
BIAS
600
μs
V
Threshold Voltage at Monitor
INTERFACE SIGNALS
TX_DISABLE Input High
TX_DISABLE Input Low
V
Figure 5
1.14
2.0
1.3
-70
1.39
REF
V
V
V
HI
V
R
= 45kꢀ (typ)
0.8
15
LO
PULL
V
V
= V
HI
CC
TX_DISABLE Input Current
μA
= V
-140
0.4
LO
GND
TX_FAULT Output Low
Shutdown Output High
Shutdown Output Low
Sinking 1mA, open collector
Sourcing 100μA
V
V
V
V
- 0.4
CC
Sinking 100μA
0.4
Note 1: AC characterization is performed using the circuit in Figure 2 using a PRBS 223 - 1 or equivalent pattern.
Note 2: Specifications at -40°C are guaranteed by design and characterization.
Note 3: Excluding I
and I
. Input data is AC-coupled. TX_FAULT open, SHUTDOWN open.
MOD
BIAS
Note 4: Power-supply noise rejection (PSNR) = 20log (V
) / ΔV
). V
is the voltage across the 15Ω load when IN+
10 noise (on VCC
OUT
OUT
is high.
Note 5: The minimum required voltage at the OUT+ and OUT- pins is +0.75V.
Note 6: Guaranteed by design and characterization.
Note 7: Tested with 00001111 pattern at 2.7Gbps.
Note 8: DJ includes pulse-width distortion (PWD).
_______________________________________________________________________________________
3
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
Typical Operating Characteristics
(V
CC
= +3.3V, C
= 0.01µF, I
= 20mA, I
= 30mA, T = +25°C, unless otherwise noted.)
APC
BIAS
MOD
A
OPTICAL EYE DIAGRAM
OPTICAL EYE DIAGRAM
ELECTRICAL EYE DIAGRAM
MOD
(2.7Gbps, 27 - 1 PRBS, 2.3GHz FILTER)
(1.25Gbps, 27 - 1 PRBS, 940MHz FILTER)
(I
= 30mA, 2.7Gbps, 27 - 1 PRBS)
MAX3738 toc03
MAX3738 toc01
MAX3738 toc02
1310nm FP LASER
1310nm FP LASER
1pF BETWEEN OUT+
AND OUT-
r = 8.2dB
e
r = 8.2dB
e
MAX738
75mV/div
54ps/div
116ps/div
52ps/div
SUPPLY CURRENT (I ) vs. TEMPERATURE
(EXCLUDES BIAS AND MODULATION CURRENTS)
ELECTRICAL EYE DIAGRAM
ELECTRICAL EYE DIAGRAM
CC
(I
= 30mA, 3.125Gbps, 27 - 1 PRBS)
(I
= 30mA, 4.25Gbps, 27 - 1 PRBS)
MOD
MOD
MAX3738 toc04
MAX3738 toc05
60
55
3.63V
50
45
2.97V
3.3V
40
35
30
46ps/div
34ps/div
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
TEMPERATURE (°C)
BIAS-CURRENT MONITOR RATIO
vs. TEMPERATURE
PHOTOCURRENT MONITOR RATIO
vs. TEMPERATURE
MODULATION CURRENT vs. R
MODSET
90
80
70
60
50
40
30
20
10
0
90
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
88
86
84
82
80
78
76
74
72
70
1
10
100
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
-40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90
R
(kΩ)
TEMPERATURE (°C)
TEMPERATURE (°C)
MODSET
4
_______________________________________________________________________________________
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
MAX738
Typical Operating Characteristics (continued)
(V
CC
= +3.3V, C
= 0.01µF, I
= 20mA, I
= 30mA, T = +25°C, unless otherwise noted.)
APC
BIAS
MOD A
DETERMINISTIC JITTER
vs. MODULATION CURRENT
PHOTODIODE CURRENT vs. R
APCSET
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
50
45
40
35
30
25
20
15
10
5
2.7Gbps
0
0.1
1
10
100
0
10 20 30 40 50 60 70 80 90
(mA)
R
(kΩ)
I
APCSET
MOD
RANDOM JITTER
vs. MODULATION CURRENT
COMPENSATION (K) vs. R
MODBCOMP
10
1
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0.1
0.01
0.001
0.01
0.1
1
10
100
0
10 20 30 40 50 60 70 80 90
(mA)
R
(kΩ)
I
MODBCOMP
MOD
TEMPERATURE COMPENSATION vs.
(R = 500Ω)
TEMPERATURE COMPENSATION vs.
R
R
(R
= 10kΩ)
TH_TEMP MODTCOMP
TH_TEMP MODTCOMP
100
90
80
70
60
50
40
30
44
42
40
38
36
34
32
30
R
= 12kΩ
R
= 12kΩ
TH_TEMP
TH_TEMP
R
= 7kΩ
TH_TEMP
R
= 7kΩ
TH_TEMP
R
= 4kΩ
= 2kΩ
TH_TEMP
R
= 4kΩ
TH_TEMP
R
TH_TEMP
R
= 2kΩ
TH_TEMP
-10
0
10 20 30 40 50 60 70 80 90
-10
0
10 20 30 40 50 60 70 80 90 100
TEMPERATURE (°C)
TEMPERATURE (°C)
_______________________________________________________________________________________
5
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
Typical Operating Characteristics (continued)
(V
CC
= +3.3V, C
= 0.01µF, I
= 20mA, I
= 30mA, T = +25°C, unless otherwise noted.)
APC
BIAS
MOD A
TRANSMITTER ENABLE
HOT PLUG WITH TX_DISABLE LOW
MAX3738 toc17
MAX3738 toc16
V
CC
3.3V
3.3V
LOW
V
CC
MAX738
t_init = 59.6ms
FAULT
0V
HIGH
LOW
FAULT
TX_DISABLE
t_on = 23.8μs
TX_DISABLE
LOW
LOW
LASER
LASER
OUTPUT
OUTPUT
10μs/div
20ms/div
TRANSMITTER DISABLE
RESPONSE TO FAULT
MAX3738 toc18
MAX3738 toc19
V
V
CC
PC_MON
EXTERNALLY
FORCED FAULT
3.3V
LOW
FAULT
t_fault = 160ns
HIGH
91.2ns
FAULT
TX_DISABLE
LOW
TX_DISABLE
LASER
OUTPUT
LASER
OUTPUT
20ns/div
400ns/div
FAULT RECOVERY TIME
MAX3738 toc20
V
PC_MON
EXTERNALLY
FORCED FAULT
FAULT
HIGH
t_init = 58ms
HIGH
LOW
TX_DISABLE
LOW
LOW
LASER
OUTPUT
40ms/div
6
_______________________________________________________________________________________
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
MAX738
Pin Description
PIN
NAME
FUNCTION
Modulation-Current Compensation from Temperature. A resistor at this pin sets the temperature
coefficient of the modulation current when above the threshold temperature. Leave open for zero
temperature compensation.
1
MODTCOMP
2, 5, 14, 17
V
CC
+3.3V Supply Voltage
3
4
IN+
IN-
Noninverted Data Input
Inverted Data Input
Transmitter Disable, TTL. Laser output is disabled when TX_DISABLE is asserted high or left
unconnected. The laser output is enabled when this pin is asserted low.
6
7
8
9
TX_DISABLE
PC_MON
Photodiode-Current Monitor Output. Current out of this pin develops a ground-referenced voltage
across an external resistor that is proportional to the monitor diode current.
Bias-Current Monitor Output. Current out of this pin develops a ground-referenced voltage across
an external resistor that is proportional to the bias current.
BC_MON
Shutdown Driver Output. Voltage output to control an external transistor for optional shutdown
circuitry.
SHUTDOWN
GND
10, 12
11
Ground
TX_FAULT Open-Collector Transmit Fault Indicator (Table 1)
13
BIAS
OUT-
OUT+
Laser Bias-Current Output
15
Inverted Modulation-Current Output. I
flows into this pin when input data is low.
MOD
16
Noninverted Modulation-Current Output. I
flows into this pin when input data is high.
MOD
Monitor Photodiode Input. Connect this pin to the anode of a monitor photodiode. A capacitor to
ground is required to filter the high-speed AC monitor photocurrent.
18
19
MD
Connect a capacitor (C
) between pin 19 (APCFILT1) and pin 20 (APCFILT2) to set the dominant
APC
APCFILT1
pole of the APC feedback loop.
20
21
APCFILT2
APCSET
(See pin 19)
A resistor connected from this pin to ground sets the desired average optical power.
A resistor connected from this pin to ground sets the desired constant portion of the
modulation current.
22
23
24
—
MODSET
MODBCOMP
TH_TEMP
EP
Modulation-Current Compensation from Bias. Couples the bias current to the modulation current.
Mirrors I
through an external resistor. Leave open for zero-coupling.
BIAS
Threshold for Temperature Compensation. A resistor at this pin programs the temperature above
which compensation is added to the modulation current.
Exposed Pad. Solder the exposed pad to the circuit board ground for specified thermal and
electrical performance.
_______________________________________________________________________________________
7
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
VOLTAGE
V
V
CC
CC
SINGLE ENDED
V
IN+
100mV (min)
V
IN-
1200mV (max)
30Ω
30Ω
Z = 30Ω
0
OUT-
DIFFERENTIAL
(V ) - (V
)
200mV (min)
IN+
IN-
30Ω
0.5pF
2400mV (max)
MAX3738
OSCILLOSCOPE
MAX738
I
OUT+
CURRENT
Z = 30Ω
0
Z = 50Ω
0
OUT+
I
75Ω
50Ω
OUT+
I
MOD
TIME
Figure 1. Required Input Signal and Output Polarity
Figure 2. Test Circuit for Characterization
HOST BOARD
MODULE
FILTER DEFINED BY SFP MSA
TO LASER
DRIVER V
L1
1μH
SOURCE
NOISE
CC
OPTIONAL
OPTIONAL
VOLTAGE
SUPPLY
C1
0.1μF
C2
10μF
C3
0.1μF
Figure 3. Supply Filter
resistor and the equivalent series resistance (ESR) of
the laser diode should equal 15Ω. To further damp
aberrations caused by laser diode parasitic induc-
tance, an RC shunt network may be necessary. Refer to
Application Note 274: HFAN-02.0: Interfacing Maxim
Laser Drivers with Laser Diodes for more information.
Detailed Description
The MAX3738 laser driver consists of three main parts:
a high-speed modulation driver, biasing block with
ERC, and safety circuitry. The circuit design is opti-
mized for high-speed, low-voltage (+3.3V) operation
(Figure 4).
At high data rates, any capacitive load at the cathode of
a laser diode degrades optical output performance.
Because the BIAS output is directly connected to the
laser cathode, minimize the parasitic capacitance asso-
ciated with the pin by using an inductor to isolate the
BIAS pin parasitics from the laser cathode.
High-Speed Modulation Driver
The output stage is composed of a high-speed differ-
ential pair and a programmable modulation current
source. The MAX3738 is optimized for driving a 15Ω
load. The minimum instantaneous voltage required at
OUT- is 0.7V for modulation currents up to 60mA and
0.75V for currents from 60mA to 85mA. Operation
above 60mA can be accomplished by AC-coupling or
with sufficient voltage at the laser to meet the driver
output voltage requirement.
Extinction Ratio Control
The extinction ratio (r ) is the laser on-state power
e
divided by the off-state power. Extinction ratio remains
constant if peak-to-peak and average power are held
constant:
To interface with the laser diode, a damping resistor
r = (2P
e
+ P ) / (2P
- P
)
P-P
AVG
P-P
AVG
(R ) is required. The combined resistance damping
D
8
_______________________________________________________________________________________
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
MAX738
V
CC
SHUTDOWN
MAX3738
INPUT BUFFER
IN+
IN-
DATA
PATH
OUT-
OUT+
R
D
I
MOD
ENABLE
I
MOD
SHUTDOWN
TX_FAULT
SAFETY LOGIC
AND
POWER DETECTOR
I
BIAS
BIAS
ENABLE
V
CC
I
BIAS
TX_DISABLE
R
= 45kΩ
PULL
V
R
I
CC
APCSET
MD
1
V
REF
APCSET
MD
I
BIAS
PC_MON
x1/2
R
PC_MON
xTC
x268
xK
I
APCSET
I
BIAS
82
T > T
I
MD
TH
BC_MON
C
MD
R
BC_MON
T
x1
V
REF
TH_TEMP
MODTCOMP
MODSET MODBCOMP
APCFILT1
APCFILT2
R
R
R
R
MODBCOMP
TH_TEMP
MODTCOMP
MODSET
C
APC
Figure 4. Functional Diagram
Average power is regulated using APC, which keeps
constant current from a photodiode coupled to the
laser. Peak-to-peak power is maintained by compen-
sating the modulation current for reduced slope effi-
ciency (η) of laser over time and temperature:
increases with temperature. Refer to Application Note
1119: HFAN-02.2.1: Maintaining the Extinction Ratio of
Optical Transmitters Using K-Factor Control for details:
K = ΔI
/ ΔI
BIAS
MOD
This provides a first-order approximation of the current
increase needed to maintain peak-to-peak power.
Slope efficiency decreases more rapidly as tempera-
ture increases. The MAX3738 provides additional tem-
perature compensation as temperature increases past
ρ
/
MD MON
P
AVG
= I
P
P-P
= η x I
MOD
Modulation compensation from bias increases the mod-
ulation current by a user-selected proportion (K) needed
to maintain peak-to-peak laser power as bias current
a user-defined threshold (T ).
TH
_______________________________________________________________________________________
9
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
V
CC
POR AND COUNTER
60ms DELAY
I
MOD
TX_DISABLE
ENABLE
COUNTER
60ms DELAY
I
BIAS
7
100ns DELAY
ENABLE
V
CC
I
MD
1
V
V
REF
PC_MON
Q
R
S
COMP
COMP
V
CC
R
R
PC_MON
RS
LATCH
I
BIAS
82
REF
BC_MON
SHUTDOWN
TX_FAULT
BC_MON
CMOS
EXCESSIVE
APC CURRENT
SETPOINT
EXCESSIVE
MOD CURRENT
SETPOINT
TTL
OPEN COLLECTOR
Figure 5. Simplified Safety Circuit
Table 1. Typical Fault Conditions
If any of the I/O pins are shorted to GND or V
(single-point failure; see Table 2), and the bias current or the photocurrent
CC
1
exceeds the programmed threshold.
2
3
End-of-life (EOL) condition of the laser diode. The bias current and/or the photocurrent exceed the programmed threshold.
Laser cathode is grounded and photocurrent exceeds the programming threshold.
No feedback for the APC loop (broken interconnection, defective monitor photodiode), and the bias current exceeds the
programmed threshold.
4
10 ______________________________________________________________________________________
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
MAX738
Table 2. Circuit Responses to Various Single-Point Faults
CIRCUIT RESPONSE TO OVERVOLTATGE OR
SHORT TO V
CIRCUIT RESPONSE TO UNDERVOLTAGE OR
SHORT TO GROUND
PIN
CC
TX_FAULT
Does not affect laser power.
Does not affect laser power.
TX_DISABLE Modulation and bias currents are disabled.
Normal condition for circuit operation.
The optical average power increases, and a fault occurs The optical average power decreases, and the APC loop
if V exceeds the threshold. The APC loop responds by increasing the bias current. A fault state
occurs if V exceeds the threshold voltage.
IN+
IN-
PC_MON
responds by decreasing the bias current.
The optical average power decreases and the APC loop The optical average power increases and a fault occurs
responds by increasing the bias current. A fault state if V exceeds the threshold. The APC loop
occurs if V exceeds the threshold voltage.
BC_MON
PC_MON
responds by decreasing the bias current.
BC_MON
The APC circuit responds by increasing the bias current
until a fault is detected; then a fault* state occurs.
MD
SHUTDOWN
BIAS
This disables bias current. A fault state occurs.
Does not affect laser power. If the shutdown circuitry is
used, the laser current is disabled.
Does not affect laser power.
In this condition, the laser forward voltage is 0V and no
light is emitted.
Fault state* occurs. If the shutdown circuitry is used, the
laser current is disabled.
The APC circuit responds by increasing the bias current
until a fault is detected; then a fault state* occurs.
Fault state* occurs. If the shutdown circuitry is used, the
laser current is disabled.
OUT+
OUT-
Does not affect laser power.
Fault state* occurs.
Does not affect laser power.
Does not affect laser power.
Does not affect laser power.
PC_MON
BC_MON
Fault state* occurs.
I
increases until V
exceeds the threshold
exceeds the threshold
I
increases until V
exceeds the threshold
exceeds the threshold
BIAS
BC_MON
BC_MON
BIAS
BC_MON
APCFILT1
APCFILT2
voltage.
voltage.
I increases until V
BIAS
I
increases until V
BIAS
BC_MON
voltage.
voltage.
MODSET
APCSET
Does not affect laser power.
Does not affect laser power.
Fault state* occurs.
Fault state* occurs.
*A fault state asserts the TX_FAULT pin, disables the modulation and bias currents, and asserts the SHUTDOWN pin.
Safety Circuitry
The safety circuitry contains a disable input
(TX_DISABLE), a latched fault output (TX_FAULT), and
fault detectors (Figure 5). This circuitry monitors the
operation of the laser driver and forces a shutdown if a
fault is detected (Table 1). The TX_FAULT pin should
Safety Circuitry Current Monitors
The MAX3738 features monitors (BC_MON, PC_MON)
for bias current (I ) and photocurrent (I ). The
BIAS
MD
monitors are realized by mirroring a fraction of the cur-
rents and developing voltages across external resistors
connected to ground. Voltages greater than V
at
REF
be pulled high with a 4.7kΩ to 10kΩ resistor to V
as
PC_MON or BC_MON result in a fault state. For exam-
ple, connecting a 100Ω resistor to ground at each mon-
itor output gives the following relationships:
CC
required by the SFP MSA. A single-point fault can be a
short to V or GND. See Table 2 to view the circuit
CC
response to various single-point failure. The transmit
fault condition is latched until reset by a toggle or
TX_DISABLE or V . The laser driver offers redundant
CC
V
= (I
/ 82) x 100Ω
BC_MON
BIAS
V
= I
x 100Ω
PC_MON
MD
laser diode shutdown through the optional shutdown
circuitry as shown in the Typical Application Circuit.
This shutdown transistor prevents a single-point fault at
the laser from creating an unsafe condition.
External sense resistors can be used for high-accuracy
measurement of bias and photodiode currents. On-chip
isolation resistors are included to reduce the number of
components needed to implement this function.
______________________________________________________________________________________ 11
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
The laser driver automatically adjusts the bias to main-
Table 3. Optical Power Relations
tain the constant average power. For DC-coupled
PARAMETER
SYMBOL
RELATION
laser diodes:
Average Power
P
P
AVG
= (P + P ) / 2
AVG
0
1
I
= I
+ I
/ 2
AVG
BIAS
MOD
Extinction Ratio
r
e
r = P / P
e
1
0
Programming the Modulation Current with
Compensation
Determine the modulation current from the laser slope
efficiency:
Optical Power of a One
Optical Power of a Zero
Optical Amplitude
Laser Slope Efficiency
Modulation Current
Threshold Current
P
P
P = 2P
x r / (r + 1)
AVG e e
1
1
P = 2P
0
/ (r + 1)
e
0
AVG
P
P
= P - P
P-P 1 0
P-P
MAX738
ꢀ
ꢀ = P
/ I
P-P MOD
I
= 2 x P
/ η x (r - 1) / (r + 1)
MOD
AVG e e
I
I
= P
/ ꢀ
P-P
MOD
MOD
The modulation current of the MAX3738 consists of a
static modulation current (I ), a current proportion-
I
P at I I
0 TH
TH
MODS
al to I
, and a current proportional to temperature.
BIAS
The portion of I
Bias Current
(AC-Coupled)
I
I
I
+ I
/ 2
BIAS
BIAS TH
MOD
set by MODSET is established by
MOD
an internal current regulator, which maintains the refer-
ence voltage of V across the external programming
resistor. See the Modulation Current vs. R
Laser to Monitor
Transfer
REF
ꢁ
I
/ P
AVG
MON
MD
MODSET
graph in the Typical Operating Characteristics and
select the value of R that corresponds to the
Note: Assuming a 50% average input duty cycle and mark
density.
MODSET
required current at +25°C:
Design Procedure
I
= I
+ K x I
+ I
MOD
MODS
BIAS MODT
When designing a laser transmitter, the optical output is
usually expressed in terms of average power and
extinction ratio. Table 3 shows relationships that are
helpful in converting between the optical average
power and the modulation current. These relationships
are valid if the mark density and duty cycle of the opti-
cal waveform are 50%.
I
= 268 x V
/ R
MODS
REF
MODSET
| T > T
| T < T
I
I
= TC x (T - T
)
TH
MODT
MODT
TH
TH
= 0
An external resistor at the MODBCOMP pin sets current
proportional to I . Open circuiting the MODBCOMP
pin can turn off the interaction between I
BIAS
and I
:
MOD
BIAS
For a desired laser average optical power (P
) and
K = 1700 / (1000 + R
)
10%
to I
AVG
MODBCOMP
optical extinction ratio (r ), the required bias and modu-
e
If I
must be increased from I
maintain the extinction ratio at elevated temperatures,
the required compensation factor is:
to
MOD
MOD1
MOD2
lation currents can be calculated using the equations in
Table 3. Proper setting of these currents requires
ρ
knowledge of the laser to monitor transfer (
slope efficiency (η).
) and
MON
K = (I
- I
) / (I
- I
)
MOD2 MOD1
BIAS2 BIAS1
A threshold for additional temperature compensation
can be set with a programming resistor at the
TH_TEMP pin:
Programming the Monitor-Diode Current
Set Point
The MAX3738 operates in APC mode at all times. The
bias current is automatically set so average laser power
is determined by the APCSET resistor:
T
TH
= -70°C + 1.45MΩ / (9.2kΩ + R
)°C 10%
TH_TEMP
The temperature coefficient of thermal compensation
above T is set by R . Leaving the
TH
MODTCOMP
ρ
/
MD MON
P
AVG
= I
MODTCOMP pin open disables additional thermal
compensation:
The APCSET pin controls the set point for the monitor
diode current. An internal current regulator establishes
the APCSET current in the same manner as the
TC = 1 / (0.5 + R
(kΩ)) mA/°C 10%
MODTCOMP
MODSET pin. See the Photodiode Current vs. R
APCSET
graph in the Typical Operating Characteristics and
select the value of R that corresponds to the
APCSET
required current at +25°C.
I
= 1/2 x V
/ R
APCSET
MD
REF
12 ______________________________________________________________________________________
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
MAX738
Maxim Laser Drivers with Laser Diodes for more informa-
tion on AC-coupling laser drivers to laser diodes.
≤
Current Compliance (I
60mA),
MOD
DC-Coupled
The minimum voltage at the OUT+ and OUT- pins
is 0.7V.
For compliance:
V
OUT+
= V
- I
/ 2 x (R + R ) ≥ 0.75V
CC MOD D L
For:
Determine C
APC
V
= Diode bias point voltage (1.2V typ)
DIODE
The APC loop filter capacitor (C
) must be selected
APC
R = Diode bias point resistance (5Ω typ)
R = Series matching resistor (20Ω typ)
For compliance:
L
to balance the requirements for fast turn-on and mini-
mal interaction with low frequencies in the data pattern.
The low-frequency cutoff is:
D
1.1
ρ
(kHz) x (η x )
MON
C (µF) ≅ 68 / (f
APC
)
3dB
V
= V
- V
- I
x (R + R ) -
OUT+
CC
DIODE MOD
D
L
High-frequency noise can be filtered with an additional
cap, C , from the MD pin to ground.
I
x R ≥ 0.7V
BIAS
L
MD
Current Compliance (I
> 60mA),
AC-Coupled
MOD
C
MD
≅ C
/ 4
APC
The MAX3738 is designed so turn-on time is faster than
For applications requiring modulation current greater
than 60mA, headroom is insufficient from proper opera-
tion of the laser driver if the laser is DC-coupled. To
avoid this problem, the MAX3738’s modulation output
can be AC-coupled to the cathode of a laser diode. An
external pullup inductor is necessary to DC-bias the
ρ
1ms for most laser gain values (η x
). Choosing a
MON
smaller value of C
reduces turn-on time. Careful
APC
balance between turn-on time and low-frequency cutoff
may be needed at low data rates for some values of
laser gain.
modulation output at V . Such a configuration isolates
CC
laser forward voltage from the output circuitry and allows
the output at OUT+ to swing above and below the sup-
ply voltage (V ). When AC-coupled, the MAX3738
CC
modulation current can be programmed up to 85mA.
Refer to Application Note 274: HFAN-02.0: Interfacing
Interface Models
Figures 6 and 7 show simplified input and output cir-
cuits for the MAX3738 laser driver. If dice are used,
replace package parasitic elements with bondwire par-
asitic elements.
V
CC
V
CC
MAX3738
PACKAGE
0.7nH
16kΩ
PACKAGE
V
CC
OUT-
0.11pF
0.7nH
IN+
0.11pF
5kΩ
0.7nH
OUT+
0.11pF
V
CC
5kΩ
0.7nH
IN-
0.11pF
MAX3738
24kΩ
Figure 6. Simplified Input Structure
Figure 7. Simplified Output Structure
______________________________________________________________________________________ 13
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
port or sustain life, or for any other application where the
failure of a Maxim product could create a situation
where personal injury or death may occur.
Layout Considerations
To minimize loss and crosstalk, keep the connections
between the MAX3738 output and the laser diode as
short as possible. Use good high-frequency layout
techniques and multilayer boards with uninterrupted
ground plane to minimize EMI and crosstalk. Circuit
boards should be made using low-loss dielectrics. Use
controlled-impedance lines for data inputs, as well as
the module output.
Exposed-Pad (EP) Package
The exposed pad on the 24-pin TQFN provides a very
low thermal resistance path for heat removal from the
IC. The pad is also electrical ground on the MAX3738 and
should be soldered to the circuit board ground for proper
thermal and electrical performance. Refer to Maxim
Application Note 862: HFAN-08.1: Thermal Considerations
of QFN and Other Exposed-Paddle Packages at
www.maxim-ic.com for additional information.
MAX738
Laser Safety and IEC 825
Using the MAX3738 laser driver alone does not ensure
that a transmitter design is IEC 825 compliant. The
entire transmitter circuit and component selections must
be considered. Each customer must determine the level
of fault tolerance required by their application, recogniz-
ing that Maxim products are not designed or authorized
for use as components in systems intended for surgical
implant into the body, for applications intended to sup-
Chip Information
TRANSISTOR COUNT: 3754
PROCESS: SiGe/BiPOLAR
14 ______________________________________________________________________________________
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
MAX738
Typical Application Circuit
+3.3V
OPTIONAL SHUTDOWN
CIRCUITRY
+3.3V
0.1μF
0.01μF
+3.3V
IN+
15Ω
10Ω
CDR
100Ω
OUT-
OUT+
0.1μF
IN-
R
R
MODBCOMP
MODTCOMP
MAX3738
MODBCOMP
MODTCOMP
TH_TEMP
BIAS
MD
FERRITE BEAD
R
TH_TEMP
C
MD
C
APC
REPRESENTS A CONTROLLED-IMPEDANCE TRANSMISSION LINE.
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
PACKAGE TYPE
24 TQFN-EP
PACKAGE CODE
DOCUMENT NO.
21-0139
T2444-3
______________________________________________________________________________________ 15
155Mbps to 4.25Gbps SFF/SFP Laser Driver
with Extinction Ratio Control
Revision History
REVISION REVISION
PAGES
CHANGED
DESCRIPTION
NUMBER
DATE
0
1/04
Initial release.
—
All
1
Changed the data sheet from 1Gbps to 2.7Gbps to 155Mbps to 2.7Gbps.
Added the lead-free package to the Ordering Information table.
MAX738
1
7/05
In the Electrical Characteristics table, added 622Mbps and 155Mbps conditions
to the deterministic jitter parameter.
2
Added the 100ꢀ resistor to the Typical Application Circuit.
13
Increased the maximum data rate 2.7Gbps to 4.25Gbps.
All
Added 4Gbps to the Applications for Fibre Channel SFF/SFP and GBIC
transceivers.
1
2
Changed the derate factor for continuous power dissipation in the Absolute
Maximum Ratings from 20.8mW/°C to 27.8mW/°C.
2
12/08
Added new TOCs 4 and 5 to the Typical Operating Characteristics section.
4
9
Changed V
to V
in Figure 4.
REF
BG
Changed the transistor count from 1184 to 3754 in the Chip Information section.
14
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
16 ____________________Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600
© 2008 Maxim Integrated Products
is a registered trademark of Maxim Integrated Products, Inc.
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